In recent years, microwave antennas have been widely used in both communication and radar applications. Due to their wide use, microwave antennas have been the subject of much attention. In particular, patch antennas, especially those capable of circular polarization operation, have been heavily researched and studied. A number of theories and methods have been proposed for constructing a patch antenna capable of generating and receiving circularly polarized signals.
For example, U.S. Pat. No. 4,903,033 issued to Tsao et al. discloses a dual polarization microwave antenna capable of generating circularly polarized signals. This antenna comprises a radiating patch, a ground plane having crossed slot apertures placed under the radiating patch, two feeding circuits, and two ports. This reference shows two ways in which circular polarization can be achieved: (1) through the use of a meanderline polarizer; or (2) through the use of a hybrid coupler. According to the first method, a meanderline polarizer is imposed onto the radiating patch such that the meanderlines are offset substantially 45 degrees with respect to each of the slot apertures. The meanderline polarizer operates to convert dual orthogonal linearly polarized signals into circularly polarized signals. The resulting antenna using this method may be quite thick and bulky, however, because meanderline polarizers need to have a thickness of at least three quarters of the wavelength at the operating frequency. At an operating frequency in the L-band region, the polarizer may need to be as thick as 9 inches. This makes for an undesirably large microwave antenna. According to the second method, the two ports of the antenna may be attached to two branches of a hybrid coupler. The coupler serves to induce a 90 degree phase difference between the input signals to the two ports, thereby producing the 90 degree phase shift necessary for circular polarization operation. Although this configuration is effective, it is not favored because it requires the use of a hybrid coupler. Hybrid couplers are difficult to fabricate using integrated circuit fabrication techniques. Consequently, they add cost and complexity to the production of the antenna.
Iwasaki et al., "A Circularly Polarized Microstrip Antenna Using a Crossed-Slot Feed," 1990 IEEE Antennas and Propagation Symposium Digest Volume II, Dallas, Tex., May 7-11, 1990, pp. 807-810, describes a radiating patch antenna for generating circularly polarized signals having a ground plane with orthogonal crossed slot apertures. A feed circuit couples an input signal to the radiating patch through the intersection point of the two apertures and causes two orthogonal linearly polarized signals to be generated. The lengths of the apertures are specifically designed to be different such that their resonant frequencies are different. At a frequency between the resonant frequencies of the two apertures, the phase of one of the linearly polarized signals lags the phase of the other signal by 90 degrees. As a result, a circularly polarized signal is generated at that particular frequency. Although this method is effective for generating circularly polarized signals, it is difficult to design an antenna operable at a specific frequency using this method. The frequency at which circular polarization is achieved cannot be calculated with much precision. Consequently, it is necessary to adjust the lengths of the apertures a number of times before a working model is obtained. Each time an adjustment is made, a new ground plane has to be produced. This can become rather tedious and expensive. A more desirable antenna would be one in which the operable frequency can be adjusted and fine-tuned without having to produce a new ground plane with each adjustment.
Askun et al., "Theory and Experiment of Electromagnetically Excited Microstrip Antennas for Circular Polarization Operation," 1989 IEEE AP-S International Symposium Digest, Volume II, San Jose, CA, June 26-30, 1989, pp. 1142-1145, describes another method for achieving circular polarization operation wherein a ground plane having a single slot aperture is placed under a rectangular, preferably square radiating patch. The radiating patch is coupled to a feeding circuit through the slot aperture. Circular polarization operation is achieved by properly adjusting the following parameters: (1) the dimensions of the radiating patch and the slot aperture; (2) orientation of the slot with respect to the patch; and (3) the position of the slot relative to the patch. This method suffers from the same shortcomings as the above methods: namely, each time an adjustment is made, a new antenna has to be built. Designing an antenna operable at a specified frequency using this method would be difficult and costly.
Askun et al., "Circular Polarization Operation of Double-Slot Fed Microstrip Antennas," 1989 AP-S International Symposium Digest, Volume II, San Jose, CA, June 26-30, 1989, pp. 640-643, describes two other methods for attaining circular polarization operation. According to the first method, a ground plane having two orthogonal, non-intersecting slot apertures is placed beneath a radiating patch. Each of the slot apertures couples the radiating patch to a different branch of a hybrid coupler. The hybrid coupler provides the 90 degree phase shift necessary for circular polarization operation. As discussed above, however, hybrid couplers are not favored as means for producing circularly polarized signals. According to the second technique of this reference, circular polarization operation can be attained without the use of a hybrid coupler. A ground plane, having two slot apertures which intersect each other orthogonally at one of their respective ends, is placed beneath a radiating patch. The patch is electromagnetically coupled to a coaxial line through the intersection point of the two apertures. A single signal on the coaxial line excites the radiating patch and causes the production of two linearly polarized orthogonal mode signals. By properly adjusting the dimensions of the patch, the dimensions of the slot apertures, and the location of the slots relative to the patch, it is possible to cause one of the produced signals to lag the other by 90 degrees, thereby creating a circularly polarized signal. Like the other antennas discussed above, however, it is difficult to design this antenna to operate at any particular frequency.
U. S. Pat. No. 4,843,400 issued to Tsao et al. describes another slot coupled antenna for generating circularly polarized signals. In the Tsao patent, a radiating patch is coupled to a feeding circuit through an elongated slot aperture. The radiating patch may take the shape of an ellipse or a near square. Depending upon the type of radiating patch used, the slot aperture is positioned such that it lies substantially along one of the diagonals of the near square patch or such that it makes a 45 degree angle with both the major and minor axes of the ellipse. The strategic placement of the slot relative to the patch causes the generation of two orthogonal components of electromagnetic energy. By experimentally adjusting the dimensions of the patch and the dimensions of the slot aperture, it is possible to cause the phase of one of the generated signals to lag the other by 90 degrees; thus, circular polarization operation is achieved. This antenna is difficult to design, however
Other references are U.S. Pat. Nos. 4,755,821 and 4,847,625.
Thus, although a number of microwave antennas exist which are capable of circular polarization operation, none are altogether satisfactory. Most are difficult to design while others require the use of clumsy polarizers and hybrid couplers. Therefore, there is a need for an improved circular polarization microwave antenna.